A known developing device for forming a thin layer of magnetic brush on a developing roll is disclosed in Japanese Patent Laid-Open No. 43037/1979. In particular, this developing device has a sleeve incorporating a rotatable magnetic roll. A layer thickness-limiting member consisting of a magnetic substance is disposed over the sleeve such that a minute gap is maintained between them. Thus, a thin layer of magnetic brush is formed on the developing roll. Japanese Patent Laid-Open No. 43038/1979 discloses a method of forming a thin layer of magnetic brush by pressing a layer thickness-limiting member against a sleeve incorporating a rotatable magnetic roll. The limiting member is made of a resilient member. The present applicant has already proposed a method of limiting developer in Japanese Patent Laid-Open No. 203975/1985. Specifically, a magnetic roll producing repulsive magnetic fields is mounted inside a rotatable sleeve. The magnetic roll has magnetic poles which are located opposite to a limiting member and have the same polarity. Hence, the magnetic forces produced from these poles repel each other. The strength of the magnetic force at the exit of the limiting portion is made larger than the strength of the force at the entrance of the limiting portion. Japanese Patent Laid-Open No. 234177/1987 discloses a developing device in which magnets producing repulsive magnetic fields are mounted inside a rotatable sleeve. A limiting means has magnetic members disposed opposite to the magnets.
The method disclosed in the above-cited Japanese Patent Laid-Open No. 43037/1979 makes use of a monocomponent magnetic toner. A magnetic curtain is formed between the layer thickness-limiting plate and the magnetic poles of the magnetic roll to control the amount of magnetic toner transferred, or the thickness of the layer. The limiting plate consists of a magnetic member disposed immediately above and close to the magnetic poles of the magnetic roll. In this method, the amount of the transferred magnetic toner is affected greatly by the spacing between the magnets and the layer thickness-limiting plate and, therefore, the print quality is affected materially by the limiting plate itself and by the accuracy at which the plate is set up. Accordingly, in order to form a toner layer of a uniform thickness on the developing roll and to maintain high print quality, it is necessary to accurately set the spacing axially of the developing roll.
The method disclosed in the above-cited Japanese Patent Laid-Open No. 43038/1979 uses a monocomponent magnetic toner. The layer thickness-limiting member consisting of a resilient member is pressed against the sleeve incorporating the rotatable magnetic roll to control the amount of the transferred magnetic toner or the thickness of the layer. This method has the disadvantage that the amount of the transferred toner varies as the resilient member ages and wears down. As a result, the print quality is varied.
The present applicant's method disclosed in the above-cited Japanese Patent Laid-Open No. 203975/1985 exploits a monocomponent magnetic developer. The magnetic forces at the position opposite to the limiting member repel each other because of the same polarity. The strength of the magnetic force at the exit of the limiting portion is made larger than the strength of the magnetic force at the entrance. Thus, the developer is prevented from collecting on the upstream side of the flow of the developer. Also, nonuniform transfer of the developer is prevented at the time of trimming. Furthermore, when the chains of the developer particles are caused to stand by the repelling magnetic forces, the limiting member cuts the chains of the developer particles. Hence, the amount of the developer adhering to the sleeve can be reduced. However, the positional relation between the limiting member and the magnetic poles producing the repulsive magnetic fields is not taken into account in this method. Therefore, the amount of the developer adhering to the sleeve can increase, depending on this positional relation. This increase results in variations in the amount of transfer.
The method disclosed in the above-cited Japanese Patent Laid-Open No. 234177/1987 uses a dual component developer. A magnetic curtain is formed between the repulsive magnetic poles of the magnetic roll and the limiting member to obtain a uniform thin layer of toner. The limiting member has the magnetic member disposed immediately above and close to the magnetic poles. A region is formed in which the developer does not adhere to the sleeve to permit the developer to stay. Also, the passage of the developer or carrier is prevented. Only the toner is allowed to adhere electrostatically to the sleeve. Thus, the toner is carried. In this method, the force with which the toner adheres to the sleeve varies, depending on the triboelectricity produced between the sleeve and the toner. For this reason, the sleeve provides a quite small surface area for the toner. That is, the toner is unable to possess a sufficient amount of electric charge and so it is difficult to form a uniform layer of toner on the sleeve. Consequently, the homogeneity of the developed image concentration and variations in the concentration against background present problems. Also, where the spacing between the sleeve and the limiting member varies, the carrier particles easily flow out. As a result, the photoconductor is damaged mechanically in the development region. In addition, the carrier particles are developed together with the toner. In this case, white unprinted portions tend to occur. Where a uniform thin layer of developer is formed instead of forming only a layer of toner, it is necessary to accurately set the spacing axially of the developing roll, in the same way as in the method of the above-described Japanese Patent Laid-Open No. 43037/1979. Further, when the developer is conveyed, the thickness of the layer of the developer is limited while repeating the formation of the chains of the developer particles and the cutting of the developer particle chains. In consequence, the developer particle chains are not cut at a given location. In this way, the thickness of the layer of the developer is not uniform.